Method and apparatus for uplink synchronization maintenance with p2p communication in wireless communication networks

ABSTRACT

A method for maintaining uplink synchronization in P2P (Peer to Peer) communication, performed by a user equipment in wireless communication systems, comprising: negotiating with a wireless communication network system about the schedule and parameters for controlling the uplink synchronization of the user equipment by the procedure of establishing P2P direct link; transmitting testing signals to the network system via a customized uplink channel according to the negotiated parameters; receiving the control information transmitted via a customized downlink channel from the network system according to the negotiated schedule; and maintaining uplink synchronization of the user equipment in P2P communication according to the control information. Uplink synchronization can be realized through the customized channel, so the present invention can overcome the problem of overload brought by implementing uplink synchronization through the downlink common control channel and system performance degradation caused by increasing the transmitting power of the user equipment to transmit P2P signals.

FIELD OF THE INVENTION

The present invention relates to a method and apparatus with P2Pcommunication in wireless communication networks, and more particularly,to a method—and apparatus for uplink synchronization maintenance withP2P communication in wireless communication networks.

BACKGROUND ART OF THE INVENTION

In conventional cellular mobile communication systems, a UE (userequipment) has to communicate with the other UE only through therelaying of base stations regardless of the distance between the twoUEs. FIG. 1 illustrates the conventional communication mode, where UE1and UE2 interact through the UTRAN consisting of the base stationtransceiver (namely Node B) and the Radio Resource controller (RNC), andthis communication mode Is also called UP-UTRAN-DOWN mode. However, insome cases when the distance between two UEs who are camping in the samecell is very close, it can be a more reasonable way for them tocommunicate directly, rather than through the relaying of base stations.This method is the so-called peer-to-peer communication, abbr. as P2P.

FIG. 2 illustrates a P2P communication mode, as FIG. 2 shown, the dashedline represents signaling link, the solid line for data link and thearrowhead for direction of information flow. Only signaling link existsbetween the UTRAN and the UE, while only data link exists between thetwo communicating UEs. Assume that only resource for maintaining basiccommunication is needed. A direct link is taken as one unit of radioresource (with fixed frequency, timeslot and spreading code), it can beeasily drawn that P2P communication mode only needs two units of radioresource to maintain basic communication. If additional signaling costfor management is ignored, P2P communication can save about 50% radioresource than conventional communication mode. Meanwhile, the UTRANstill holds control over P2P communication, especially over how to useradio resources, so that network operators can easily charge for theradio resources used by P2P communication.

A method and apparatus for establishing P2P communication in wirelesscommunication networks, as described in the patent application entitled“A Method and Apparatus for Establishing P2P Communication in WirelessCommunication Networks”, filed by KONINKLIJKE PHILIPS ELECTRONICS N.V.on Mar. 7, 2003, Attorney's Docket No. CN030003, application Ser. No.03119892.9, is suitable to any TDD CDMA communication system includingTD-SCDMA systems, and incorporated herein as reference.

A method and apparatus for radio link establishment and maintenance withP2P communication in wireless communication networks, as described inthe patent application entitled “A Method and Apparatus for Radio LinkEstablishment and Maintenance with P2P Communication in WirelessCommunication Networks”, filed by KONINKLIJKE PHILIPS ELECTRONICS N.V.on Mar. 7, 2003, Attorney's Docket No. CN030005 and application Ser. No.03119895.3, is suitable to any TDD CDMA communication system includingTD-SCDMA systems, and incorporated herein by reference.

After establishing uplink synchronization with the UTRAN through thesame random access procedure as existing TD-SCDMA systems, the UE canestablish a P2P direct link with the other UE, in accordance with themethod and apparatus as described in the application document whoseapplication Ser. No. is 03119892.9, i.e.: allocate relevant dedicatedresource for two P2P UEs. Then, direct link between the two UEs can beestablished and maintained in accordance with the method and apparatusas described in the application whose application Ser. No. is03119895.3, so that the two UEs can receive and transmit P2P signals inthe allocated timeslots respectively, and thus P2P communication betweentwo UEs can be implemented.

However, when P2P communication mode is applied in TDD wirelesscommunication systems, if a P2P UE shares the same uplink timeslot asother UEs employing UP-UTRAN-DOWN mode, it may potentially impair uplinksynchronization and thus deteriorate the system performance.Specifically, when any UE working in P2P communication mode moves, thedistance between the UE and the UTRAN will change, which means the UEestablishing uplink synchronization with the UTRAN via random accessprocedure in cell search procedure after powering on, is likely to losesynchronization with the UTRAN. At this time, corresponding methods areneeded to keep the UE in synchronization with the UTRAN. Otherwise, theUTRAN can overhear information on the P2P link between the two UEsduring P2P communication process. In this way, if it can't be guaranteedthat the P2P signals from the two UEs arrive at the UTRAN at the sametime, the spreading code demodulated at the UTRAN won't be orthogonal,thus the system performance is deteriorated.

But P2P communication mode changes the UP-UTRAN-DOWN communication modein conventional TD-SCDMA systems, i.e.: in conventional TD-SCDMAsystems, the UTRAN is involved in any ongoing communication asinformation source or destination or relayer/forwarder, thus it canmonitor and control the UE's uplink signal transmitting time through aspecific traffic burst structure, so as to maintain uplinksynchronization between the UE and the UTRAN in connect mode. Whileduring P2P communication, the UE has no dedicated traffic channel linkedwith the UTRAN, so it can only overhear information between two P2P UEs,which means that the UTRAN can't maintain uplink synchronization withconventional mode by adjusting the uplink synchronization TA (timeadvance) with a specific traffic burst structure even if it can overhearand estimate the communicating UE's synchronization shift information.Therefore, for communication systems like TD-SCDMA systems that have astrict requirement for uplink synchronization, how to maintain uplinksynchronization between a moving UE and the UTRAN, will be a greatchallenge.

An method and apparatus for maintaining uplink synchronization betweenthe moving UE and the UTRAN to be controlled by sending synchronizationshift information via the paging control channel in the downlink commoncontrol channel between the UE and the UTRAN, is described in the patentapplication entitled “A Method and Apparatus for Uplink SynchronizationMaintenance with P2P Communication in Wireless Communication Networks”,filed by KONINKLIJKE PHILIPS ELECTRONICS N.V. on Mar. 7, 2003,Attorney's Docket No. CN030004, application Ser. No. 03119895.3, andincorporated herein as reference.

In the method and apparatus for uplink synchronization maintenance asdescribed in the application, with no direct dedicated channel betweenthe UTRAN and the UEs, the PCCH (paging control channel) in existingdownlink common control channel is utilized to send synchronizationshift information to the UEs, thus radio resource is saved. But themethod and apparatus brings two new problems: first, when the UTRANcan't overhear information transferred on the P2P direct communicationlink, it will require the UEs to increase transmission power for P2Psignals, which violates the transmission power requirement for P2Pcommunication; second, sending synchronization shift information byusing downlink common control channel, will potentially cause overloadon downlink common control channel. Detailed description will be givenbelow to the two problems:

First, the method and apparatus for uplink maintenance as described inthe application document with application Ser. No. as 03119894.5,estimates the synchronization shift information of two UEs, according tothe UTRAN's overheard signal on the direct link between the two P2P UEs.Then, the synchronization shift information obtained through estimationis sent to the two UEs via downlink common control channel between theUTRAN and the UEs. However, when failing to overhear informationtransferred on P2P direct communication link, the URAN has to send pollmessage to the UE to increase its power to transmit P2P signals till itcan overhear information transferred over P2P direct communication link.This means transmission power of the P2P communicating UEs is expectedto transmit signals to each other with transmission power high enough toreach the UTRAN, otherwise, the UTRAN may fail to overhear signals onP2P direct link and therefore fail to maintain uplink synchronizationfor each UE. On the other side, In accordance with the purpose of P2Pcommunication establishment, P2P link generally is established betweentwo UEs who are very close to each other, and in order to satisfy thesame quality for the same services, the P2P communicating UEs areexpected to transmit P2P signals to each other with much lowertransmission power than that between the UE s and the UTRAN. With lowertransmission power, P2P communication can benefit the system withreduced system interference and saved battery consumption, and thereforeimprove system capacity. As described above, the UE's transmission powermay be increased for the UTRAN to overhear signals on the P2P link,which imposes contradicts with increasing the system performance andcapacity by using lower transmission.

Second, in accordance with the method and apparatus for uplinksynchronization maintenance as described in the application whoseapplication Ser. No. is 03119894.5, when overhearing signals transferredon the direct link between two P2P communicating UEs, the UTRAN can getthe synchronization shift information by estimating the midamble in thetraffic burst structure. Then, the synchronization shift information issent to the P2P communicating UEs via PCH on downlink control channel,to adjust the UEs' TA for transmitting P2P signals, and thus the UE andthe UTRAN can be kept synchronized. According to TD-SCDMA protocolstandard, we know a specific PCH is shared by several UEs and theassociated PCH for each UE is Identified by the calculation result ofits IMSI (International Mobile Station Identification, only the lastthree digitals is used in this case), DRX (Discontinuous Reception)cycle and the number of common control channels. Each UE has a differentIMSI of its own, while DRX cycle and the number of common controlchannel are determined by the network system configuration. For each UEsharing the same PCH, all calculation results for identifying each UE'sPCH and calculated according to the IMSI, DRX cycle and the number ofcommon control channels, will fall in the same PCH block in pagingcycle. If there are many UEs camping in the same cell and some of themhappen to work in P2P communication mode and share the same PCH, thesynchronization maintenance will avoidably add great overload to thePCH, especially when the UTRAN fails to overhear the ongoing signals onthe P2P links in order to keep uplink synchronization of those UEs. Theoverloaded PCH may potentially block normal paging from outside of thecell and impair the system performance in the mobile communicationsystem.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method and apparatusfor uplink synchronization maintenance with P2P communication inwireless communication systems. By employing customized channels betweenthe UTRAN and the UEs, the method and apparatus can control the P2Pcommunicating UEs to maintain uplink synchronization, thus effectivelyresolve the above problems like overload of PCH caused by controllingthe P2P communicating UEs to maintain uplink synchronization via commoncontrol channels and system performance degradation caused by increasingthe UE's transmission power when the UTRAN can't overhear P2P linksignals.

To achieve the object of the present invention, a method is proposed foruplink synchronization with P2P (Peer to Peer) communication, performedby the UE in a wireless communication system, comprising: (a)negotiating with the wireless communication network system about theschedule and parameters for controlling uplink synchronization for theUE, during P2P direct link establishment; (b) transmitting testingsignals to the network system via a customized uplink channel accordingto the negotiated parameters, with the TA and transmission powerobtained through random access procedure; (c) receiving the controlinformation transmitted via a customized downlink channel from thenetwork system according to the negotiated schedule; (c) maintaininguplink synchronization for the UE in P2P communication according to thecontrol information. Wherein the customized uplink channel and downlinkchannel are customized by the networks system for the UE between the UEand the network system.

An uplink synchronization maintenance method for a P2P UE to be executedby the network system in a wireless communication system in the presentinvention, comprising: (i) customizing an uplink channel and a downlinkchannel between the network system and the UE for the UE; (ii)negotiating with the UE about the schedule and parameters forcontrolling uplink synchronization; (iii) receiving testing signals fromthe UE via the customized uplink channel, according to the negotiatedparameters; (iv) estimating the control information for uplinksynchronization for the UE, according to the received signals; and (v)transmitting the control information to the UE via said customizeddownlink channel according to the negotiated schedule, so as to controlthe UE to maintain uplink synchronization with the network system duringP2P communication procedure.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

FIG. 1 is a schematic diagram illustrating conventional UP-UTRAN-DOWNcommunication mode;

FIG. 2 is a schematic diagram illustrating the P2P communication mode inaccordance with the present invention;

FIG. 3 illustrates the chart flow for the method for the uplinksynchronization maintenance with P2P communication in TD-SCDMA systemsin accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Regarding to the method for a UE to maintain uplink synchronization withthe UTRAN during P2P communication provided in the present invention,its essence lies in: maintaining customized channels between each UE andthe UTRAN to control each UE's uplink synchronization when two UEs areperforming P2P communication, wherein the customized uplink channel isfor the UTRAN to estimate synchronization shift information and thecustomized downlink for the UTRAN to send the adjusted synchronizationshift information to the corresponding UE.

The channels to be customized between the UTRAN and the P2P UEs may beDCH (Dedicated channel) or USCH/DSCH (Uplink/Downlink shared channel),or any other traffic channel or control channel satisfying therequirement. During P2P direct link establishment procedure, thesecustomized channels are allocated to corresponding UEs. During uplinksynchronization maintenance procedure in P2P communication, thecustomized channels can also be adjusted. Different from existingdedicated channels and USCH/DSCH, these customized channels can be usedto control the P2P UEs to maintain uplink synchronization, according tospecific UE requirement with specific sub-frame and occupancy interval.The uplink synchronization maintenance method provided in the presentinvention, reserves customized channels between the UTRAN and the UEs,but it has obvious difference from the uplink synchronizationmaintenance method in conventional TD-SCDMA systems, which can becharacterize in: (1) traffic information is not transferred on thecustomized channels between the UTRAN and the corresponding UE, sotraffic information transferred on the P2P direct link won't producedirect influence upon these customized channels. But in conventionalTD-SCDMA systems, dedicated channels between the UTRAN and the UEs arefor transferring traffic information; (2) a UE in P2P communication modecan send N continuous traffic bursts to the UTRAN, so as to estimate theuplink synchronization shift information, i.e.: the UTRAN can estimatethe synchronization shift information according to N continuous trafficbursts, but in the uplink synchronization control method in conventionalTD-SCDMA systems, the UTRAN estimates the synchronization shiftinformation according to a traffic burst transmitted by the UE; (3)these channels between the UTRAN and corresponding UEs are customizedaccording to different UE requirement, which means the corresponding UEscan share these channels with other UEs and the corresponding UEs onlyoccupy these channels intermittently and regularly when needed, so theUTRAN can estimate uplink synchronization intermittently and regularlyaccording to the negotiated schedule. But in conventional TD-SCDMAsystems, the UTRAN estimates the timing information continuously byevaluating the channel impulse response of each midamble in the sametimeslot used by users in each sub-frame.

A detailed description below will go to the uplink synchronizationmaintenance method as proposed in the present invention via thecustomized channels between the UTRAN and the UEs, taking TD-SCDMAsystem as example, in conjunction with FIG. 3.

First, the same as the cell search phase in conventional communication,the UE searches for a cell to camp in after powering on (step S1). TheUE establishes uplink synchronization with the UTRAN through randomaccess procedure (step S2). Afterwards, the UTRAN customizes uplink anddownlink channels for the UE when the UE establishes P2P link with theother UE (step S3), wherein the customized uplink and downlink channelscan be the DCH or USCH/DSCH, and these customized channels shouldn'tconflict with the radio resource used for P2P direct communication.

During P2P link establishment, the UE and the UTRAN negotiate theschedule and parameters for uplink synchronization control, that is: todetermine the interval to execute uplink synchronization control and howmany continuous bursts to be used in uplink synchronization estimation.According to the negotiated result, the UE and the UTRAN set TimerD (fordetermining the interval to execute uplink synchronization) and counterV(for determining the number of continuous bursts) respectively (stepS4). During uplink synchronization maintenance procedure, the value ofthe timer and the counter can be adjusted according to P2P link status,for instance, according to the changing communication environment,distance between the UE and the other UE who are performing P2Pcommunication, locations of the two UEs, their transmission power andspecific requirements from the UTRAN or the two UEs (e.g. the UTRAN maywant to get the status information of all P2P direct links). Generallyspeaking, when two P2P UEs get closer, the needed transmission powerwill be lower and thus their potential impact upon the UTRAN as uplinkinterference will be smaller, and hence, the interval of Timer isexpected to increase and the number of continuous bursts is expected todecrease. In opposite situations such as the distance is getting furtheror radio link is getting worse, the needed transmission power increasesaccordingly. At this time, the potential impact upon the UTRAN, i.e.uplink interference, increases correspondingly, thus the interval ofTimer is expected to decrease and the number of continuous bursts to beset by the counter Is expected to increase accordingly.

After the timer and the counter for uplink synchronization control areset according to the negotiated parameters, the P2P UE sets its TA fortransmitting P2P signals via the direct traffic channel according to thesynchronization adjustment acquired during random access procedure inwhich uplink synchronization is established (step S5). Then, the UEbegins to communicate directly with the other UE via the P2Pcommunication link.

While setting its TA for transmitting P2P signals, the UE sets its TAand transmission power to transmit testing signals for controllinguplink synchronization via the above customized uplink channel to theUTRAN, according to the synchronization adjustment and power controlinformation obtained when establishing uplink synchronization with theUTRAN during random access procedure (step S6).

During P2P communication, the UE checks whether it is time to sendsignals for uplink synchronization (step S7). If the timer doesn'texpire, the UE go on with direct communication (step S11). If the timerexpires, the UE sends traffic bursts to the UTRAN continuously via thecustomized uplink channel (step S8), and at the same time the counterbegins to count (step S9). The UE judges whether the traffic bursts sentreach the predefined value of the counter (step S10). If not reached,the UE keeps sending traffic bursts (step S8); if reached, the UE stopssending traffic bursts to the UTRAN. It should be pointed here that nodata are contained in the traffic bursts sent by the UE to the UTRAN,but P2P link status information can be included if needed. When sendingN traffic bursts to the UTRAN, the UE continues P2P directcommunication. After sending N traffic bursts, the UE waits for thedownlink control information from the UTRAN via the customized downlinkchannel, according to the negotiated schedule.

According to the predefined schedule, the UTRAN receives and records theN expected continuous traffic bursts transferred from the UE via thecustomized uplink channel. If the UTRAN receives M of the N trafficbursts (1<M<=N), the UTRAN estimates and obtains the uplinksynchronization shift information SS and power control information PC,by evaluating M continuous channel impulse responses of each midambleused in the customized uplink channel (step S12). If P2P link statusinformation is included in the traffic bursts sent via the customizeduplink channel, the UE determines whether the counter and timer need tobe adjusted according to evaluating the P2P link status information(step S13). The UTRAN sets the UE's synchronization shift informationSS, power control information PC, the adjusted values of the timerand/or the counter, and then sends the estimated synchronization shiftinformation SS, power control information PC, the adjusted values of thetimer and/or the counter to the UE, in the next available downlinktimeslot via the customized downlink channel (step S14). If the UTRANfails to receive any traffic burst from the UE in the above step S12, itonly sends power control command and doesn't send synchronization shiftinformation and the adjusted values of the timer and/or counter to theUE in step S14, so that the UE can increase its expected transmissionpower the next time it sends N traffic bursts via the customized uplinkchannel.

According to the negotiated schedule, the UE receives the controlinformation from the UTRAN via the customized downlink channel, whereinthe control information includes the above synchronization shiftinformation SS, power control information PC, the adjusted values of thecounter and/or the timer. Then, the UE determines whether the TA fortransmitting signals needs to be adjusted, according to thesynchronization shift information SS (step S15). Wherein, If informationSS is included in the control information, that is, the UE is out ofsynchronization with the UTRAN, the UE adjusts its transmission time inP2P direct link in step of ±k/8 chips (step S16), wherein the defaultvalue of k (from 1 to 8) is to be determined through negotiation duringP2P link establishment and the value of k can be adjusted during thecall. If information SS is not included in the control information, thatis, the UE maintains synchronization with the UTRAN or the UTRAN can'tevaluate the synchronization shift, the UE won't adjust its transmissiontime for P2P signals. Meanwhile, the UE determines whether to adjust itstransmission time and transmission power to be used for transmittingsignals on the customized uplink channel next time (step S17). Ifneeded, the UE adjusts its transmission time and transmission power tobe used for transmitting signals on the customized uplink channel nexttime, according to the received synchronization shift information SS andpower control information PC as above (step S18). Moreover, the UEfurther determines whether to adjust the value of timer and/or thecounter (step S19). If needed, the UE adjusts the interval to sendsignals to the UTRAN and the number of continuous traffic bursts nexttime via the customized uplink channel, according to the adjusted valueof the timer and/or the counter in the received control information(step S20). After making the above adjustments, the UE continues P2Pcommunication, and iterates the procedures in the above steps from S7 toS20 during P2P communication.

The above method for uplink synchronization maintenance with P2Pcommunication in wireless communication networks in accordance with thepresent invention, can be implemented in computer software, or incorresponding hardware, or in combination of software and hardware. Inan embodiment of the present invention, the hardware structures ofexisting mobile terminals and the UTRAN, remain unchanged, while theabove steps from S1 to S20 can all be implemented in software modules.

BENEFICIAL RESULTS OF THE INVENTION

As described above, compared with the method for controlling uplinksynchronization by utilizing downlink common control channels, themethod for uplink synchronization provided in the present invention Isobviously different. The difference lies in that there exist customizeduplink channel for estimating synchronization shift information anddownlink channel for sending the synchronization shift informationbetween the UE and the UTRAN, so the UTRAN needn't overhear trafficsignals transferred on the P2P link in the method of the presentinvention. Meanwhile, the present invention realizes uplinksynchronization control through customized channels between the UE andthe UTRAN, so problems like overload of downlink common control channeland system performance degradation caused by increasing the UE'stransmission power, can be overcome. Thus, signals from different UEscan arrive at the UTRAN at the same time during P2P communication, whichmaintains the orthogonal integrity of modulated spreading code of uplinksignals and cancels the interference of the P2P direct link to thewireless systems.

Compared with the method for controlling uplink synchronization byutilizing downlink common control channels, the method for uplinksynchronization provided in the present invention occupies a little moreradio resource, but UEs only use these customized channelsintermittently and the resource cost added by these customized channelscan be shared by P2P dynamic channel allocation procedure, P2P softswitching procedure, power control procedure and etc, which is necessaryfor P2P communication systems to mange radio resource. Therefore, themethod provided in the present invention can still save radio resourcethan existing communication systems.

It is to be understood by those skilled in the art that the method foruplink synchronization maintenance with P2P communication as proposed inthis invention, is also applicable to other TDD mode, TDD SCDMA andadvanced wireless communication systems that have certain requirementfor uplink synchronization.

It is also to be understood by those skilled in the art that the methodfor uplink synchronization maintenance as disclosed in this inventioncan be modified considerably without departing from the spirit and scopeof the invention as defined by the appended claims.

1. A method performed by a mobile terminal having wireless wide areanetwork (WWAN) and wireless local area network (WLAN) interfaces, themethod comprising the steps of: (a) detecting presence of a WWAN and aWLAN; (b) establishing a WWAN call between the mobile terminal as acalling mobile terminal and a called mobile terminal via a WWAN; (c)sending extra information including a handover request together with anetwork address of the calling mobile terminal to the called mobileterminal, for initiating a WLAN call with the called mobile terminal viaa WLAN, if the WLAN is present; (d) receiving a response from the calledmobile terminal; and (e) if the response indicates an acceptance of thehandover request, establishing the WLAN call with the called mobileterminal via the WLAN.
 2. The method of claim 1, further comprising thestep of, after step (e), releasing the WWAN call.
 3. The method of claim1, further comprising the steps of: during the WLAN call, detectingwhether the calling mobile terminal is moving away from the WLAN; and ifthe calling mobile terminal is moving away from the WLAN, establishing anew WWAN call with the called mobile terminal via the WWAN.
 4. Themethod of claim 1, further comprising the step of: detecting a preferredradio interface for communication based on a user profile in the callingmobile terminal; wherein step (c) is performed if the preferred radiointerface is the WLAN interface and the WLAN is present.
 5. The methodof claim 1, further comprising a step of: accepting a WLAN callconnection with another mobile terminal via a WLAN, as a receivingmobile terminal; during the WLAN call, detecting whether the receivingmobile terminal is moving away from the WLAN; and if the receivingmobile terminal is moving away from the WLAN, sending a handover requestto the other mobile terminal for initiating a new WWAN call with theother mobile terminal via a WWAN.
 6. A method performed by a callingmobile terminal having dual radio interfaces respectively for first andsecond wireless communication networks, the method comprising the stepsof: (a) detecting presence of the first and second wirelesscommunication networks; (b) establishing a first call between the mobileterminal as a calling mobile terminal and a called mobile terminal via afirst wireless communication network; (c) sending extra informationincluding a handover request together with a network address of thecalling mobile terminal in the second wireless communication network tothe called mobile terminal for initiating a second call with the calledmobile terminal via the second wireless communication network, if thesecond wireless communication network is present; (d) receiving aresponse from the called mobile terminal; and (e) if the responseindicates an acceptance of the handover request, establishing the secondcall with the called mobile terminal via the second wirelesscommunication network.
 7. The method of claim 6, further comprising thestep of, after step (e), releasing the first call.
 8. The method ofclaim 6, further comprising the steps of: during the second call,detecting whether the calling mobile terminal is moving away from thesecond wireless communication network; and if the calling mobileterminal is moving away from the second wireless communication network,establishing a third call with the called mobile terminal via the firstwireless communication network.
 9. The method of claim 6, furthercomprising a step of: detecting a preferred radio interface forcommunication based on a user profile in the calling mobile terminal;wherein step (c) is performed if the preferred radio interface is forthe second wireless communication network and the second wirelesscommunication network is present.
 10. The method of claim 6, furthercomprising a step of: accepting a fourth call connection with anothermobile terminal via the second wireless communication network, as areceiving mobile terminal; during the fourth call, detecting whether thereceiving mobile terminal is moving away from the second wirelesscommunication network; and if the receiving mobile terminal is movingaway from the second wireless communication network, sending a handoverrequest to the other mobile terminal for initiating a fifth call withthe other mobile terminal via the first wireless communication network.11. The method of claim 1, wherein in step (c) the extra information issent to the called mobile terminal using a short message service (SMS)technique.
 12. The method of claim 1, wherein in step (c) the extrainformation is sent to the called mobile terminal using a watermarkingtechnique.
 13. The method of claim 12, wherein the extra information issent to the called mobile terminal via a voice channel between thecalling and called mobile terminals.
 14. The method of claim 13, whereinthe extra information is inserted in the silent durations of inputvoice.
 15. The method of claim 11, wherein the network address is anInternet Protocol (IP) address.
 16. A mobile terminal having wirelesswide area network (WWAN) and wireless local area network (WLAN)interfaces, comprising: means for detecting presence of a WWAN and aWLAN; means for establishing a WWAN call between the mobile terminal asa calling mobile terminal and a called mobile terminal via a WWAN; meansfor sending extra information including a handover request together witha network address of the calling mobile terminal to the called mobileterminal, for initiating a WLAN call with the called mobile terminal viaa WLAN, if the WLAN is present; means for receiving a response from thecalled mobile terminal; and means for establishing the WLAN call withthe called mobile terminal via the WLAN, the WLAN call establishingmeans establishing the WLAN call if the response indicates an acceptanceof the handover request.
 17. The terminal of claim 16, furthercomprising means for releasing the WWAN call, the releasing meansreleasing the WWAN call after the WLAN call is established.
 18. Theterminal of claim 16, further comprising: means for detecting, duringthe WLAN call, whether the calling mobile terminal is moving away fromthe WLAN; wherein the WWAN call establishing means establishes a newWWAN call with the called mobile terminal via the WWAN, if the callingmobile terminal is moving away from the WLAN.
 19. The terminal of claim16, further comprising: means for detecting a preferred radio interfacefor communication based on a user profile in the calling mobileterminal; wherein the sending means sends the extra information, if thepreferred radio interface is the WLAN interface and the WLAN is present.20. The terminal of claim 16, further comprising: means for accepting aWLAN call connection with another mobile terminal via a WLAN, as areceiving mobile terminal; means for detecting, during the WLAN call,whether the receiving mobile terminal is moving away from the WLAN; andmeans for sending a handover request to the other mobile terminal forinitiating a new WWAN call with the other mobile terminal via a WWAN,the sending means of the receiving mobile terminal sending the handoverrequest if the receiving mobile terminal is moving away from the WLAN.21. The terminal of claim 16, wherein the extra information is sent tothe called mobile terminal using a short message service (SMS)technique.
 22. The terminal of claim 16, wherein the extra informationis sent to the called mobile terminal using a watermarking technique.23. The terminal of claim 22, wherein the extra information is sent tothe called mobile terminal via a voice channel between the calling andcalled mobile terminals.
 24. The terminal of claim 22, wherein thesending means includes: a silence control circuit that receives inputvoice and detects silent periods in the input voice, the control circuitoutputting a control signal upon detecting a silent period; a modulationcircuit that modulates the extra information into modulated analogsignals and is responsive to the control signal, the modulation circuitoutputting the modulated analog signals upon receiving the controlsignal; and a time division multiplexer that multiplexes the modulatedanalog signals into the silent periods of the input voice.
 25. Theterminal of claim 22, wherein the sending means includes: a silencecontrol circuit that receives input voice and detects silent periods inthe input voice, the control circuit outputting a control signal upondetecting a silent period; a watermarking circuit that receives theextra information and is responsive to the control signal, thewatermarking circuit outputting the extra information upon receiving thecontrol signal; and a time division multiplexer that multiplexes theextra information into the silent periods of digitized input voice. 26.The terminal of claim 20, wherein the accepting means includes: afrequency detection circuit that detects pre-defined frequency patternsrepresenting digital symbols from received voice data; and ademodulation circuit that demodulates the frequency patterns to output acorresponding digital sequence as the extra information.
 27. Theterminal of claim 20, wherein the accepting means includes awatermarking detection circuit that detects a pair of pre-defineddigital sequences to recover a digital sequence enclosed by the pair asthe extra information.
 28. The terminal of claim 21, wherein the networkaddress is an Internet Protocol (IP) address.
 29. A mobile terminalhaving dual radio interfaces respectively for first and second wirelesscommunication networks, comprising: means for detecting presence of thefirst and second wireless communication networks; means for establishinga first call between the mobile terminal as a calling mobile terminaland a called mobile terminal via a first wireless communication network;means for sending extra information including a handover requesttogether with a network address of the calling mobile terminal in thesecond wireless communication network to the called mobile terminal forinitiating a second call with the called mobile terminal via the secondwireless communication network, if the second wireless communicationnetwork is present; means for receiving a response from the calledmobile terminal; and means for establishing the second call with thecalled mobile terminal via the second wireless communication network,the second call establishing means establishing the second call if theresponse indicates an acceptance of the handover request.
 30. Theterminal of claim 29, further comprising means for releasing the firstcall, the releasing means releasing the first call after the second callis established.
 31. The terminal of claim 29, further comprising: meansfor detecting, during the second call, whether the calling mobileterminal is moving away from the second wireless communication network;wherein the first call establishing means establishes a third call withthe called mobile terminal via the first wireless communication network,if the calling mobile terminal is moving away from the second wirelesscommunication network.
 32. The terminal of claim 29, further comprising:means for detecting a preferred radio interface for communication basedon a user profile in the calling mobile terminal; wherein the sendingmeans sends the extra information if the preferred radio interface isfor the second wireless communication network and the second wirelesscommunication network is present.
 33. The terminal of claim 29, furthercomprising: means for accepting a fourth call connection with anothermobile terminal via the second wireless communication network, as areceiving mobile terminal; and means for detecting, during the fourthcall, whether the receiving mobile terminal is moving away from thesecond wireless communication network; and means for sending a handoverrequest to the other mobile terminal for initiating a fifth call withthe other mobile terminal via the first wireless communication network,the sending means of the receiving mobile terminal sending the handoverrequest if the receiving mobile terminal is moving away from the secondwireless communication network.
 34. The terminal of claim 29, whereinthe extra information is sent to the called mobile terminal using ashort message service (SMS) technique.
 35. The terminal of claim 29,wherein the extra information is sent to the called mobile terminalusing a watermarking technique.